As a technology for increasing a friction on ice (i.e., ice traction) of rubber sought in tread rubber of studless tires, the technique of compounding heat-expandable microcapsules or heat-expandable graphite into a diene-based rubber has already been devised (see Japanese Patent Publication (A) No. 10-316801, Japanese Patent Publication (A) No. 11-035736 and Japanese Patent Publication (A) No. 13-279020). However, there is a problem in that further improvement of ice traction of rubber has been demanded and the increase in the hardness of tread rubber due to aging causes, for example, a decrease in the performance in the second year or thereafter.
In this regard, Japanese Patent Publication (A) No. 2003-327745 discloses a rubber composition comprising a rubber matrix, into which capsule particles enclosing a water repellent substance (e.g., silicone oil etc.) are compounded. However, the capsules do not thermally expand, and, therefore, the improvement in the ice traction is not sufficient. Further, Japanese Patent Publication (A) No. 2003-105138 describes production of a rubber composition containing heat-expandable microcapsules during which coating the surface of the heat-expandable microcapsules in advance with an oily substance, however, the problem of the decrease in performance causes by the increase in hardness due to aging of the tread rubber remain unsolved.